When your logs are flooded with statements like „System: The update of the antivirus scanner definition file failed due to a download error. Check your Internet connection.“ you will find no perfect solution online.
Some tried to restart the NAS and it seemed to work for some days. Others deactivated and reactivated the virus scanner option.
But it might simply be a memory problem that occurs when downloading the virus patterns. The NAS uses ClamAV, the antivirus software licensed under the GPL.
What worked for me was to activate the SSH access option to log into the NAS and work on the command line. Running „freshclam“ in the foreground went south with a message about not enough memory available.
Running „top“ showed me that „clamscan“ and „clamd“ consumed the most memory so I killed the processes for the moment to have more memory available for the next step.
The definition files are stored in the directory /var/lib/clamav/ . I renamed both „main.cld“ and „daily.cld“ which are about 300 Megabytes in size each (I could have simply deleted them).
The next attempt to run freshclam worked. After all files were loaded I stopped freshclam with Ctrl-C and restarted the NAS.
Let’s see if the downloads start to work again the next days.